It separated into different layers because each has different densities and they sit on top of each other.

"What do you mean by density?" asked Mr. B.

It's how packed the particles are.

This one is thick so it's on the bottom. This one is thinnest.

Doesn't oil have lighter density than water?

If we put a thicker liquid in, it would go to the bottom.

There's more of this one that's on the bottom.

The atoms in some are heavier than the ones in others.

Mr. B. realized how many different ways the students explained what they saw, for example, thickness and thinness, heaviness and lightness, more and less, different densities and atoms. The discussion gave him a sense for what the students were thinking. It was clear to him that the investigations he had planned for the following weeks to focus more closely on density would be worthwhile.

Mr. B. divided the class into seven groups of four the next day. On each of the group's tables were small cylinders. Mr. B warned the students not to drink the liquid. Each group was to choose one person to be the materials manager and one to be the recorder as they proceeded to find out what they could about the same liquids used the day before (all of which were available on the supply table). Only the materials manager was to come to the supply table for the liquids, and the recorders kept track of what they did. Forty minutes later, Mr. B. asked students to clean up and gather to share their observations.

Every group identified some of the liquids. The water was easy, as was the vegetable oil. Some students knew corn syrup, others recognized the detergent. Several groups combined two and three liquids and found that some of them mixed together, and others stayed separate. Some disagreements arose about which liquid floated on which. Mr. B. suggested that interested students come back during their lunch time to try to resolve these disagreements. One group replicated the large cylinder, shook it vigorously, and was waiting to see whether the liquids would separate. Mr. B. asked that group to draw what the cylinder contents looked like now, put it on the windowsill, and check it the next day.

Mr. B. began the third day with a large density column again. This time he gave a small object to each of four students—a piece of wood, aluminum, plastic, or iron. He asked the class to predict what would happen when each of the four objects was released into the column. The students predicted and watched as some objects sank to the bottom, and others stopped somewhere in the columns.

"What do you think is going on?" asked Mr. B. "How can you explain the way these objects behaved? I don't want answers now," he went on, "I want you to try out some more things yourselves and then we'll talk.'' He then divided the class into four groups and gave each a large density column with the liquid layers. The students worked in their groups for 30 minutes. The discussion was animated as different objects were tried: rubber bands, a penny, a nickel, a pencil,